It has become increasingly desirable to reduce the amount of paper required in the volume mailing of information such as bills and financial statements. As such, increasing emphasis has been placed upon the construction of "one-piece mailers" in which a single sheet of paper receives textual and other graphic information and then is subsequently folded to become a self-contained integral envelope. In order to secure the folded envelope for mailing, an adhesive material is usually applied in a separate step around the free edges. Upon folding, these edges overlap and a subsequent sealing step utilizing pressure, moisture, solvents or heat, among other influences, occurs. The particular adhesive is sensitive to a given type of sealing process.
Often, however, the adhesive is too strong to be pulled apart and, thus, owing to the integral structure of the mailer, in order to prevent damage to the textual part of the sheet, tearable perforations are subsequently cut around the sealed edges to allow ease of opening for revealing the information. Such perforations, however, result in waste of paper.
Another disadvantage of the traditional one-piece mailer sea sealing process is that it is generally difficult to apply glue or other adhesives after text has been added to the sheet. Thus, pregluing of each sheet at predetermined locations is necessary. The placement of the text must then be tailored to conform to the glue pattern. This tends to limit the types of text that may be included and requires significant advance planning of paper sheet size and text layout as well as storage of a large inventory of preglued sheets. As such, inventory investments become large while versatility of sheet and text patterns remains small.
Furthermore, since many adhesives are designed to activate when exposed to moisture, heat or pressure, they may be highly prone to aging. Thus, over time, the adhesive placed upon stored sheets may lose some or all of its sealing properties as the external environment activates it. In some instances, this activation may even cause stacked sheets to become permanently joined together, ruining valuable supplies of sheets. The risk of adhesive activation may, therefore, require additional expenses for more careful storage and climate control of sheet inventories.
It may also be desirable to create windows through a part of a sheet in order to expose interior text (such as an address) eliminating the need for individual marking the outer face of the sheet. In conventional envelope designs, a window is often covered with an attached transparent glassine layer to enhance the strength and protection of the envelope relative to the underlying text. An open window with no covering may be prone to tearing since it may be easily pulled upwardly away from the inside text sheet. The envelope would, thus, prove significantly more secure if the edges of the window could be tacked down to the text carrying paper. However, adhesives are difficult to apply and seal in central areas and may damage the text.
Another disadvantage of preglued sheets is that they are more difficult to store in a large roll and to subsequently apply print to and to cut prior to sealing. The roll storage system may cause the same difficulties as found in individual stacked sheets and printing has to be accurately registered with respect to each adhesive line ahead of time or print on the entire roll of sheets may become off centered.